Method and device for on-line regeneration of a signal transmitted by wavelength division multiplexed solitons and optical telecommunication system comprising such a regenerating device

ABSTRACT

The invention enables wavelength division multiplexed (WDM) solitons conveyed by an optical fiber to be regenerated synchronously. The invention consists in using optical delay lines, in particular of the photorefractive filter type, prior to the regenerators so as to resynchronize a subset of the m soliton channels, and in placing the regenerator at a position where the n-m other soliton channels are naturally synchronized.

The invention relates to the field of telecommunications by opticalfiber, and more particularly to telecommunications over long distances.For very long distance optical fiber links, such as transoceanic links,it is known to use a signal of the so-called “soliton” type havingspecial spectral properties that enable the signal to propagate alongthe dispersive fiber without appreciable chromatic dispersion, i.e.advantage is taken of the way refractive index depends on the intensityof the signal to counterbalance chromatic dispersion, or vice versa. Thespectral shape of the signal is preserved in spite of the effects ofpropagation distance, which are thus mainly concerned with line losses.Line losses can be compensated by in-line optical amplification, e.g. bymeans of erbium-doped fiber amplifiers (EDFAs).

When transmitting solitons with in-line amplification (EDFA) theproblems that remain to be solved are known:

1) Gordon-Haus jitter which causes uncertainty concerning the arrivaltimes of the bits of the signal; and

2) the accumulation of noise that comes from the optical amplifiersamplifying spontaneous emission.

One solution to that problem is put forward in EP-A-6 576 208. In thatdocument, a plurality of filters having various center frequencies areinserted at points along a link for transmitting solution type signals,thereby enabling the solitons to be amplified periodically withoutamplifying spontaneous emission noise exponentially. In that system, thesolitons are not regenerated. According to that document, an advantageof such a system is that it is compatible with transmission in the formof a wavelength division multiplex (WDM).

Synchronous modulation for regenerating solitons in-line is described inthe document: “10 Gbit/s soliton data transmission over one millionkilometers” by Nakazawa et al., published in Elect. Lett., 27 (14), pp.1270-72, Jul. 4, 1991.

That document teaches using an LiNO₃ optical modulator to performsynchronous modulation of solitons, with a clock signal generated usingthe same clock as that used for the soliton source. A very long distancelink was simulated using a 500 km long fiber loop with an erbium-dopedfilter optical amplifier every 50 km and with regeneration once per tripround the loop. Because of the dispersion of the soliton transmissionfiber, which lies in the range −0.7 ps/km/nm to −2.2 ps/km/nm with amean value of −1.5 ps/km/nm, the travel time for making one trip roundthe loop depends on the wavelength of the soliton. That is why such asystem is incompatible with WDM transmission, as emphasized in theabove-mentioned document EP-A-0 576 208 (cf. page 2, lines 21-24).

Other documents in the state of the art relate to WDM type opticallinks.

For example, the document “Wavelength division multiplexing withsolitons in ultra-long distance transmission using lumped amplifiers” byL. F. Mollenauer et al., published in Journal of Lightwave Tech., 9 (3),pp. 362-367, March 1991, proposes a system for transmitting WDM solitonswith periodic optical amplification for transoceanic distances (9000km). The teaching of that document relates mainly to collisions betweensolitons of different wavelengths. That document gives typical valuesfor various parameters in such a link so as to limit the Gordon-Hausjitter that stems from interaction between adjacent channels.Nevertheless, in all the cases considered in that document, synchronousarrival of solitons at the end of the link is neither provided norrequired.

The various aspects of managing dispersion in non-regenerated WDMsystems are also considered in the document “Dispersion managements onsoliton transmission in fibers with lumped amplifiers” by S. Kumar etal., published in Proc. Int'l. Symposium on Physics and Applications ofOptical Solitons in Fibers, Kyoto, Japan, pp. 1-12, Nov. 14-17, 1995(cf. last chapter of the document).

Thus, on reading the documents of the prior art, it can be seen thatoptical links having a plurality of wavelength division multiplexedchannels do not enable solitons to be regenerated since the channels arenot synchronous. In this context, the question of synchronousregeneration is therefore beside the point.

That is why, starting from well-established prejudices of the personskilled in the art it would not appear possible to envisage very highdata rate WDM optical links over very long distances using wavelengthdivision multiplexed solitons and regeneration for eliminatingGordon-Haus jitter and for maintaining the optimum spectral shape of thesolitons.

An object of the invention is to mitigate the drawbacks of the priorart.

To this end, the invention provides apparatus for regenerating anoptical signal in the form of a bit stream represented by solitonsdefined in particular by a propagation wavelength and a bit rate, saidapparatus comprising a clock recovery circuit for extracting a clocksignal from said optical signal and an optical modulator forregenerating said solitons, and being characterized in that it includes,upstream from the modulator, synchronization means for synchronizingsolitons emitted on n channels having respective different wavelengths,where n>1, said channels and said different wavelengths being associatedwith different group times, said synchronization means having m opticaldelay lines, where 1≦m≦n, the delay τ_(i) for the line i, where 1≦i≦m,being selected in such a manner as to compensate for the differencesbetween the group times associated with the various channels.

In an advantageous embodiment, the synchronization means have m opticaldelay lines, where m≦n, the delay τ_(i) for channel i, where 1≦i<m,being selected in such a manner as to compensate for the group timedifferences between m channels, and also have at least one line withoutoptical delay for the n-m other channels.

In particular, the synchronization means comprise a single line withoutoptical delay, said line without optical delay being designed to receivemultiplexed solitons emitted over a plurality of channels.

In a presentlt preferred, first embodiment, the synchronization meansincludes an optical line fitted with m photorefractive filters inseries, the frequency of each filter being associated with the frequencyof a respective channel, and the respective position of each filter i,where 1≦i≦m, being selected so as to produce said delay τ_(i) for thesolitons emitted on channel i; control means for applying the solitonsreceived by the synchronization means to said optical line and forapplying the solitons reflected by the filters of said optical line toan outlet port of the synchronization means; and an optical coupler forconveying the solitons emitted on the n-m channels which are notassociated with a filter to the outlet port of the synchronizationmeans. In this embodiment, the control means is a three-part opticalcirculator.

In a second embodiment of the invention, the synchronization meanscomprises a demultiplexer, a set of m lines in parallel, each includinga length of optical delay line, a multiplexer, and at least one linewithout optical delay disposed between the demultiplexer and themultiplexer.

In a third embodiment of the invention, the synchronization meanscomprise: a divider; a set of m lines in parallel each having arespective filter for selecting one channel, and a length of opticaldelay line; a concentrator; and at least one line without optical delaybetween the divider and the concentrator, said line without delay havinga filter for selecting at least one channel.

The invention also provides an optical transmission system for conveyingsignals each of which is in the form of a bit stream represented bysolitons, which solitons are defined in particular by a propagationwavelength and by a bit rate, said transmission system comprising atleast an emitter and a receiver interconnected by an optical fiber, saidsystem including at least one optical regenerator apparatus of theinvention.

Advantageously, in such an optical transmission system, each regeneratorapparatus is disposed at a distance Z_(R) from said emitter or from thepreceding regenerator apparatus, where the distance Z_(R) is selected insuch a manner that its product with the arrival time differenceδτ_(g)=τ_(g)(λ₁)·τ_(g)(λ_(l)) satisfies the following condition:

[kT−T/a]<δτ_(g)Z_(R)<[kT+T/a]

where: k is an integer; a≧4; T is the bit time; Z_(R) is in km; dt_(g)is in ps.km⁻¹; and λ₁ and λ_(l) are the end wavelengths of the spectrumband defined by said subset of n-m channels.

In a particularly advantageous manner, the clock recovery circuitextracts from the optical signal a signal of wavelength λ_(k) lying inthe range λ₁ to λ_(l), such that τ_(g)(λ_(k)).Z_(R)=kT.

The characteristics and advantages of the invention appear more clearlyfrom the following description of embodiments given by way ofnon-limiting illustration and with reference to the accompanyingdrawings, in which:

FIG. 1 is a block diagram of an optical fiber optical transmissionsystem of the invention;

FIG. 2 is a block diagram of regenerator apparatus of the invention;

FIG. 3 shows a first embodiment of the synchronization means of theinvention, designed for resynchronizing the solitons emitted by the nchannels of an n-channel WDM link;

FIG. 4 shows a second synchronization means of the invention designed toresynchronize the solitons emitted by the n channels of an n-channel WDMlink; and

FIG. 5 shows a third embodiment of the synchronization means of theinvention, designed for resynchronizing the solitons emitted by the nchannels of an n-channels WDM link.

In all the figures, the same numerical references refer to the sameelements, and the figures are not necessarily to scale for reasons ofclarity.

One solution to the problem of synchronizing solitons emitted overdifferent channels is described in French patent application No.96/00732 filed on Jan. 23, 1996 in the name of Alcatel SubmarineNetworks and entitled “Méthode et dispositif de régénération en ligned'un signal transmis par solitons multiplexés en longueur d'onde via lamodulation synchrone et systeme de télécommunications optiques utilisantla méthode” [A method and apparatus for in-line regeneration bysynchronous modulation of a signal transmitted by wavelength divisionmultiplexed solitons, and an optical telecommunications system using themethod ], the content of which is incorporated herein by reference.

The synchronization principle developed in that prior patent applicationis summarized below insofar as it is necessary for understanding thepresent invention. The reader is invited to refer to the prior patentapplication for more detail.

In that prior patent application, it is proposed to take account of theoptical path lengths travelled by the solitons as a function of theirwavelengths, so that the solitons are synchronized, at leastapproximately, at the location where the modulator is located.

Because of chromatic dispersion, the soliton signals emitted on thevarious channels lose synchronization as they propagate along an opticalfiber. However, all of the signals are periodic and have identical bitrates on emission. This gives rise to solitons in adjacent channels“colliding” at various points along the transmission line (seetheoretical description in the above-cited article by Mollenauer etal.). As a result, and given the frequency offsets between the variouschannels, all of the channels are synchronized with one another atcertain points that are regularly spaced apart along the line. Theproposal is to determine what this spacing is and to place regeneratorsat such points of synchronization, for the purpose of performingsynchronous modulation using a single modulator and withoutdemultiplexing.

For example, for two channels transmitted at λ₁ and λ₂, where λ₀ is thezero dispersion wavelength, and for D1=λ₂-λ₁, the arrival timedifference at the modulator results from the group time differenceδτ_(g)=τ_(g)(λ₂)-τ_(g)(λ₁), i.e.: $\begin{matrix}{{\delta\tau}_{g} = {\frac{1}{2}\left( \frac{D}{\lambda} \right)_{\lambda_{o}}\left\{ {\left( {\lambda_{2} - \lambda_{0}} \right)^{2} - \left( {\lambda_{1} - \lambda_{0}} \right)^{2}} \right\}}} \\{= {\frac{1}{2}\left( \frac{D}{\lambda} \right)_{\lambda_{o}}\left\{ {{\Delta\lambda}^{2} + {2\Delta \quad {\lambda \left( {\lambda_{1} - \lambda_{0}} \right)}}} \right\} \left( {{ps} \cdot {km}^{- 1}} \right)}}\end{matrix}$

when the slope (dD/dλ)_(λ0) is non-zero, and otherwise δτ_(g)=D.(λ₂-λ₁).

The method proposed thus consists in using a single synchronousmodulator for all of the WDM channels by carefully choosing the distancebetween the transmitter and the first modulator, or between successivemodulators, on the basis of the spacing between the channels and thechromatic dispersion of the fiber, so as to ensure that all of thechannels are synchronized wherever they pass through a modulator. Inpractice, the distance Z_(R) between modulators is selected so that thegroup time difference δτ_(g) satisfies the condition:

[kT−T/a]<δτ_(g)Z_(R)<[kT+T/a]

where k is an integer, a≧4, and T is the bit time (for Z_(R) in km, anddt_(g) in ps.km⁻¹). This constraint makes it possible to obtainapproximate synchronization between two WDM channels having wavelengthsλ₁ and λ₂. Better synchronization can be obtained by reducing the widthof the time windows (i.e., by increasing a), until the desired degree ofsynchronization is obtained.

The technique proposed in that prior patent application is implementedin the context of the present invention. More precisely, in atransmission system having n WDM channels, a modulator is placed at alocation where a subset of n-m, where m<n, channels are naturallysynchronized, using the technique described in the prior patentapplication, and the remaining m channels are brought intosynchronization with the subset of channels.

This combination of two synchronization techniques presents clearadvantages: the constraint on positioning the modulator is less severethan in the prior patent application since it is no longer necessary forall of the channels to be naturally synchronized (with increasing numberof WDM channels, the distance Z_(R) between two successive points whereall of the channels are naturally synchronized increases, therebyincreasing the attenuation, the dispersion, etc. of the signals), andthe synchronization means can be simpler since it need only process aportion of the channels.

FIG. 1 is a diagram of an example of an optical fiber opticaltransmission system suitable for conveying and regenerating a WDMoptical signal made up of solitons. The system comprises an opticalfiber F, an optical emitter E, at least one regenerator device RG, aplurality of in-line optical amplifiers G1, G2, . . . , GK, . . . , aplurality of channel filters FC1, FC2, . . . , FCk, . . . , and anoptical receiver R. The optical emitter E comprises a plurality ofoptical sources suitable for emitting solitons at respective frequenciesλ₁, λ₂, . . . , λ_(n) and a multiplexer M for injecting the solitonsinto the optical fiber F. Symmetrically, the receiver comprises ademultiplexer D and a plurality of optical detectors suitable forreceiving the respective solitons at frequencies λ₁, λ₂, . . . , λ_(n).The optical amplifiers are distributed, preferably in regular manner,along the line to compensate for the attenuation to which the solitonsare subjected. In conventional manner, the optical amplifiers may be ofthe erbium-doped fiber amplifier (EDFA) type. The channel filters FC1,FC2, . . . , FCk, . . . are located downstream from the opticalamplifiers G1, G2, . . . ; they serve to reduce the time width of thesolitons, and thus to reduce time jitter. The term “channel filter” isused to designate a bandpass filter which passes a plurality of narrowbands of different center frequencies corresponding to the wavelengthsof the various channels in the wavelength division multiplexed system.

An optical transmission system as shown in FIG. 1, but lacking in-lineregenerator apparatus, is to be found in the state of the art, cf. theabove-cited articles by L. F. Mollenauer et al.

The present invention lies specifically in the fact of providing in-lineregeneration for an optical signal of the WDM soliton type. FIG. 2 is ablock diagram of regenerator apparatus of the invention.

The regenerator apparatus RG comprises synchronization means 2 and amodulator 4. The modulator 4 is a conventional modulator and is used forregenerating a soliton type signal at a signal frequency, i.e. a non-WDMsignal. Such a modulator is described in particular in the above-citedarticle by Nakazawa et al. It comprises an optical modulator MOD, e.g.of the LiNO₃ type, for performing synchronous modulator of solitons,which modulator is controlled by an electronic control signal producedby a clock circuit on the basis of the in-line soliton signal. The clockrecovery means comprise an optical coupler C3 for extracting a fractionof the optical signal, a clock extraction circuit CLKX, a delay line forproviding a delay DEL, and an amplifier GM for delivering the controlpower needed to operate the LiNO₃ modulator MOD.

The modulation means may include birefringent polarization controldevices PC. Such devices may equally well be provided after the channelfilters (FIG. 1).

For simultaneous synchronous modulation of a plurality of wavelengthmultiplexed soliton signals to be possible, i.e., for it to be possibleto modulate signals at different wavelengths, having different groupvelocities, and thus different travel times, it is necessary for thesolitons emitted in the various channels to be synchronous.

Various embodiments of regenerator apparatus of the invention aredescribed below.

A presently preferred synchronization means is shown diagrammatically inFIG. 3. It comprises an optical circulator 6 having three ports P1, P2,P3, an optical line having an optical fiber 8 with m photorefractivefilters FPR1, FRP2, . . . , FRPm that reflect at the wavelengths, λ₁,λ₂, . . . ,λ^(m) (m<n) respectively, and an optical coupler 20 forconveying the n-m other channels which are not reflected by thephotorefractive filters to the outlet port of the synchronization means.The optical circulator is designed to apply a signal received an itsport P1 to its port P2, and a signal received on its port P2 to its portP3, The ports P1 and P3 constitute respectively the inlet and the outletof the synchronization means 2. It will thus be understood that thesoliton signal emitted in frequency channel λ_(i), where 1≦i≦n, reachesport P1, is taken to port P2, travels along fiber 8 to a filter FPRi,where it is reflected back to the circulator 6, and is finally taken toport P3. The relative positions of the photoreflective filters areselected in such a manner as to compensate for the delays between thesignals on channels 1 to m relative to the signals on channels m+1 to n,where the latter signals are naturally synchronized (the modulator beingplaced at a location where those channels are synchronous). The delayscan be determined as follows.

For each channel (wavelength λ), the group time (per km) is given by therelationship:${\tau_{g}(\lambda)} = {{\tau_{g}\left( \lambda_{o} \right)} + {\frac{1}{2}\left( \frac{D}{\lambda} \right)_{\lambda_{o}}\left( {\lambda - \lambda_{0}} \right)^{2}}}$

where D is the mean dispersion of the transmission fiber and λ₀ is thewavelength for zero dispersion. At the inlet to the regenerator, thegroup time difference δτ_(k1)=τ_(g)(λ_(k))-τ_(g)(λ_(n)) between channeln taken as a reference channel and channel k is thus given by:$\begin{matrix}{{\delta\tau}_{\kappa 1} = {{\tau_{g}\left( \lambda_{k} \right)} - {\tau_{g}\left( \lambda_{n} \right)}}} \\{= {\frac{1}{2}\left( \frac{D}{\lambda} \right)_{\lambda_{n}}\left\{ {\left( {\lambda_{k} - \lambda_{0}} \right)^{2} - \left( {\lambda_{n} - \lambda_{0}} \right)^{2}} \right\}}} \\{= {\frac{1}{2}\left( \frac{D}{\lambda} \right)_{\lambda_{n}}\left\{ {{\Delta\lambda}_{k1}^{2} + {2{{\Delta\lambda}_{k1}\left( {\lambda_{n} - \lambda_{0}} \right)}}} \right\}}}\end{matrix}$

where Δλ_(k1)=λ_(k)-λ_(n).

The above two relationships relate to the case where the dispersionslope (dD/dλ)₈₀ ₀is non-zero. It is nevertheless possible and indeedadvantageous to make a system in which the dispersion slope isperiodically corrected (compensated) by inserting short lengths ofopposite-characteristic fiber, or indeed to make a transmission fiberwhose slope is zero or at least flattened. Regardless of whether thesystem has a dispersion slope that is compensated or that is genuinelyzero, the group time (per km) is given by the following relationship:

τ_(g)(λ)=τ_(g)(λ_(n))+D(λ-λ_(n))

where D is the (constant) dispersion in the spectral range {λ_(n), . ..λ}. The group time difference δτ_(k1)=τ_(g)(λ_(k))·τ_(g)(λ_(n)) isgiven by the relationship δτ_(k1)=D.Δλ_(k1).

If T_(bit) represents the bit time (or the synchronous modulationperiod), a number N_(k1) (for k=1, . . . , n) can be defined as beingthe integer which satisfies the following relationship for channel k:N_(k1). T_(bit)≦δτ≦(N_(k1)+1).T_(bit), i.e. N_(k1) =E(δτ_(k1)/T_(bit))where E(x) represents the integer portion of the argument of x. Theinteger N_(k1)represents the maximum bit time number included in thedelay between channel n and channel k. The quantity that matters is notthe accumulated delay between two individual bits belonging to channelsn and k, but rather the relative delay between the corresponding timewindows. The advance of window k relative to window n is thus given byΔτ_(k1) (advance)=δτ_(k1) - N_(k1).T_(bit) and the delay of the window krelative to window n is given by: $\begin{matrix}{{{\Delta\tau}_{k1}({delay})} = {T_{bit} - {{\Delta\tau}_{k1}({advance})}}} \\{= {{\left( {N_{k1} + 1} \right) \cdot T_{bit}} - {\delta\tau}_{k1}}} \\{= {\left\{ {1 - \left\lbrack {\frac{{\delta\tau}_{k1}}{T_{bit}} - {E\left( \frac{{\delta\tau}_{k1}}{T_{bit}} \right)}} \right\rbrack} \right\}.}}\end{matrix}$

Observe that for practical considerations, an arbitrary additionalquantity can be added to this delay providing the additional quantity isequal to an integer number of bit times T_(bit). When the delay isapplied to each channel, all of the signals are still synchronous at theinlet to the modulator.

The photorefractive filters are advantageously implemented in the formof Bragg filters photoetched directly in the fiber. They provide theadvantage of a high extinction ratio and they make it possible to definethe delay for each channel very accurately.

Another embodiment of the synchronization means is shown in FIG. 4. Itcomprises a demultiplexer 22 having one inlet and m+1 outlets, a set ofm+1 lines in parallel, and a multiplexer 24 having m+1 inlets and oneoutlet. Amongst the set of lines in parallel, a line 26 associated withall of the channels that are naturally synchronized does not include anyoptical delay line, while each of the m other lines is associated with arespective one of the other channels and includes a respective opticaldelay line τ_(i), where 1≦i≦m, which imparts an optical delay such thatall of the n channels are synchronized at the outlet from thesynchronization means.

Another variant embodiment of the synchronization means is shown in FIG.5. It comprises a distributor 28 having one inlet and m+1 outlets, a setof m+1 lines in parallel, and a concentrator 30 having m+1 inlets andone outlet. Amongst the set of lines in parallel, one line 32 isassociated with all of the channels that are naturally synchronized andincludes a channel filter 34 corresponding to those channels, and a setof m lines each associated with one channel and each having a respectiveoptical delay line such that all of the n channels are synchronized atthe outlet from the synchronization means. In this way, the filter 16_(i), where 1≦i≦m, of line i and the delay τ_(i) imparted by the portionof delay line in line i are selected respectively to pass the solitonsemitted on channel i and the delay said solitons by a delay τ_(i) suchthat the solitons travelling over line i are synchronized at the outletfrom the concentrator with the solitons transmitted over the line 32.

The synchronization means shown in FIG. 5 provide better extinction ofadjacent channels than the embodiment shown in FIG. 4, but this isachieved at the cost of insertion loss that increases as a function ofn^(2.) This insertion loss is nevertheless acceptable so long as thenumber n channels is small. When transmitting a frequency multiplex onn=2 or 3 channels, the embodiment of FIG. 5 is presently preferred overthe embodiment of FIG. 4.

Synchronization means other than those described with reference to FIGS.3 to 5 are known and can be used in the context of the presentinvention. For example, a selective delay per channel can be obtained byusing an optical fiber having dispersion compensation or an opticalfiber including a photorefractive chip filter (where the term chirp iswell known to the person skilled in the art and designates atransmission medium in which low frequencies propagate faster than highfrequencies).

The optical coupler used for recovering the clock signal can be placedon the transmission line as shown in FIG. 2, but it can also be placedon the optical fiber 8 (FIG. 3) or on one of the m+1 parallel lines(FIG. 4) or (FIG. 5), or at some other point of the synchronizationmeans.

The invention is not limited to the embodiments shown, but on thecontrary it extends to means equivalent to those described and to allembodiments that comply with the accompanying claims.

What is claimed is:
 1. Apparatus for regenerating an optical signal inthe form of a bit stream represented by solitons defined in particularby a propagation wavelength and a bit rate, said apparatus comprising aclock recovery circuit for extracting a clock signal from said opticalsignal and an optical modulator for regenerating said solitons, andbeing characterized in that it includes, upstream from the modulator,synchronization means for synchronizing solitons emitted on n channelshaving respective different wavelengths, where n>1, said channels andsaid different wavelengths being associated with different group times,said synchronization means having m optical delay lines, where 1≦m≦n-2,the delay τ_(i) for the line i, where 1≦i≦ m, being selected in such amanner as to compensate for the differences between the group timesassociated with various channels.
 2. Apparatus according to claim 1,characterized in that the synchronization means have m optical delaylines, where m≦n-2, the delay τ_(i) for channel i, where 1≦i<m, beingselected in such a manner as to compensate for the group timedifferences between m channels, and also have at least one line withoutoptical delay for the n-m other channels.
 3. Apparatus according toclaim 2, characterized in that the synchronization means comprise asingle line without optical delay, said line without optical delay beingdesigned to receive multiplexed solitons emitted over a plurality ofchannels.
 4. Apparatus according to claim 1, characterized in that thesynchronization means include an optical line fitted with mphotorefractive filters in series, the frequency of each filter beingassociated with the frequency of a respective channel, and therespective position of each filter i, where 1≦i≦m, being selected so asto produce said delay τ_(i) for the solitons emitted on channel i;control means for applying the solitons received by the synchronizationmeans to said optical line and for applying the solitons reflected bythe filters of said optical line to an outlet port of thesynchronization means; and an optical coupler for conveying the solitonsemitted on the n-m channels which are not associated with a filter tothe outlet port of the synchronization means.
 5. Apparatus according toclaim 4, characterized in that said control means is a three-partoptical circulator.
 6. Apparatus according to claim 1, characterized inthat the synchronization means comprises a demultiplexer, a set of mlines in parallel, each including a length of optical delay line, amultiplexer, and at least one line without optical delay disposedbetween the demultiplexer and the multiplexer.
 7. Apparatus according toclaim 1, characterized in that the synchronization means comprise: adivider; a set of m lines in parallel each having a respective filterfor selecting one channel, and a length of optical delay line; aconcentrator; and at least one line without optical delay between thedivider and the concentrator, said line without delay having a filterfor selecting at least one channel.
 8. Apparatus according to claim 1,characterized in that it includes a channel filter at the output fromthe synchronous modulator.
 9. An optical transmission system forconveying signals each of which is in the form of a bit streamrepresented by solitons, which solitons are defined in particular by apropagation wavelength and by a bit rate, said transmission systemcomprising at least and a manner and a receiver interconnected by anoptical fiber, said system being characterized in that it includes atleast one optical regenerator apparatus according to claim
 1. 10. Asystem according to claim 9, characterized in that each regeneratorapparatus is disposed at a distance Z_(R) from said emitter or from thepreceding regenerator apparatus, where the distance Z_(R) is selected insuch a manner that its product with the arrival time differenceδτ_(g)=τ_(g)(λ₁)-τ_(g)(λ_(l)) satisfies the following condition:[kT−T/a]<δτ_(g)Z_(R)<[kT+T/a] where: k is an integer; a≧4; T is the bittime; Z_(R) is in km; dt_(g) is in ps.km⁻¹; and λ₁ and λ_(l) are the endwavelengths of the spectrum band defined by said subset of n-m channels.11. A system according to claim 10, characterized by a clock recoverycircuit which extracts a signal of wavelength λ_(k) from the opticalsignal, where λ_(k) lines in the wavelength range λ₁ to λ_(l) such thatτ_(g)(λ_(k)).Z_(R)=kT.